PROCESSES: BASICS OF PROCESS DESIGN - Biomass as a Sustainable Energy Source for the Future - page 212

changes. Performing rigorous energy balancing requires a lot of physical
properties to determine the species enthalpies. In the latter case, one better uses
a standard process flow sheet simulation program, e.g., ASPEN ® , ( tinyurl.com/
mfeymzk) to solve coupled mass and energy balances and determine pressure
losses over process units.
e. Evaluation
At this level of design, the economic potential function can be refined to include
estimates of investment costs for the function blocks and utility costs for balan-
cing the heat effects (cooling and heating requirements at specified temperature
levels) and for compression duties (energy). The investment costs can be
expressed as capital charge estimates, scaled with throughput through the func-
tion blocks. It cannot be based yet on volume or size, as these quantities are not
available at this level of design. In the next design level, it will be attempted to
decrease the utility costs by heat, solvent, and water integration. Capital cost
estimates can be obtained from cost estimation software for conceptual
(front-end) process design, e.g., ASPEN Capital Cost Estimator ® , ( tinyurl.
com/nhw4msc) and from books of professional engineering organizations, as
issued by Dutch Association of Cost Engineers (DACE) (DACE, 2012) or
IChemE (Gerrard, 2000). Parallel to an economic evaluation, one can analyze
over a sequence of process units where the main losses occur in atom and energy
efficiencies.
f. Selection and (g) Documentation
The generated alternatives are documented, and the better ones are selected and
propagated to the next level of design, dealing with process integration for opti-
mal use of common resources, such as thermal heat, solvents, and water, in the
process. The process integration step is required before one can start to design
the process units. It is necessary to know which resources are in excess or short
supply in the processing functions and can be exchanged with other processing
functions or the utility system. Such mass and heat integration in a process has
often just as much impact on process economy and ecology as an improved
detailed design of an individual process unit.
7.9 EXAMPLE OF ANALYSIS AND EVALUATION IN
PROCESS DESIGN
A FT process is used as an example to perform an analysis and an evaluation of a
design at level 3. For the analysis, one needs:
i. A physical process model, to compute flows of species and some ecological
and technological performance indicators
ii. A closure of this model by means of design specifications
iii. An economic process model to compute the economic potential at level 3